Wire for high-speed electrical discharge machining

ABSTRACT

The invention concerns an electrode wire comprising an unalloyed copper core coated with a diffused zinc alloy coating layer, whereof the thickness is more than 10% of the wire diameter. The coating layer is optionally plated with a thin zinc, copper, nickel, silver or gold surface contact film. Such a wire achieves higher electrical discharge machining speed.

This application is a U.S. national phase application of PCTInternational Application No. PCT/FR02/04515 filed Dec. 20, 2002.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to electrode wires for spark erosionmachining to cut or finish electrically conductive parts.

Spark erosion is used to machine an electrically conductive part bygenerating sparks between an electrically conductive wire and the partto be machined. The electrically conductive wire moves in the lengthwisedirection near the part, and also moves progressively in the transversedirection relative to the part as a result of movement in translationeither of the wire or of the part.

The sparks progressively erode the part and the wire. The longitudinalmovement of the wire maintains at all times a wire diameter in thesparking area sufficient to prevent it breaking. The relative movementof the wire and the part in the transverse direction cuts the part ortreats its surface, as appropriate.

Spark erosion machines comprise means for holding and tensioning alength of wire in the vicinity of the part to be machined in a sparkingarea filled with a dielectric such as water, means for moving the wirelongitudinally in the sparking area, means for generating a sparkingcurrent between the wire and the part to be machined, and means forproducing relative movement of the wire and the part transversely to thelongitudinal direction of the wire.

There are at present many types of spark erosion wire, classified intotwo main families.

The wires in the first family have a generally homogeneous transversestructure, consisting of copper, brass, tungsten or molybdenum, forexample. The selected alloy must satisfy electrical conductivity andmechanical strength requirements. Conductivity is necessary to feedenergy into the sparking area. Mechanical strength is necessary toprevent the wire breaking in the sparking area. If possible, the alloyis chosen so that the wire has a behavior favorable to erosion, i.e. sothat the wire causes fast erosion. The maximum erosion speed of a wireis the speed limit beyond which the wire breaks if the sparking energyis increased in an attempt to accelerate erosion.

As a general rule, each wire structure confers a machining rate, amachining accuracy and a surface state.

Thus brass wires containing 35% to 37% zinc have been proposed, whichconstitute an economically acceptable compromise, but at the cost of arelatively low erosion speed.

The second family of spark erosion wires comprises coated wires, i.e.wires consisting of a metal core coated with a surface layer that isgenerally a homogeneous metal or alloy layer. During spark erosionmachining using these wires, the electrical arc formed through thedielectric, such as water, between the surface of the wire and thesurface of the part must not reach the center of the wire, or the wirewill break. It is the coating of the wire that is worn away.

The benefit of coated wires is that the core of the wire may be selectedas a function of its electrical and mechanical properties, and thecoating may be selected as a function of its erosion properties and itscontact resistance.

Accordingly, the document FR 2 418 699 proposes coating a copper orbrass core with an alloy of zinc, cadmium, tin, lead, bismuth orantimony. The document teaches that the coating increases the machiningrate. The example given is a copper core coated with a coatingapproximately 15 μm thick for an overall diameter of 180 μm.

However, it is sometimes found that the above kind of spark erosion wiredoes not achieve an optimum machining rate, and there is a requirementto increase further the machining rate.

In the current state of the art of spark erosion using wires comprisinga copper-based core coated with an alloy coating, it has always beenrecommended that copper alloys or microalloys be used to improve themechanical properties of the wires. The benefit of improving themechanical properties of the core, and thus the mechanical properties ofthe wire as a whole, is very important for obtaining straight wires thatcan be passed through spark erosion machines without straighteningannealing, and it has always been considered that this also reduces therisk of the wire breaking during spark erosion.

Specifications of copper alloys and microalloys are in particularpublished in a document entitled “Les propriétés du cuivre et de sesalliages”, Centre d'Information du Cuivre, Laitons et Alliages, Paris,1992. Those properties are reproduced in the table below:

0.2% proof stress (in MPa) for AFNOR state H14 or after IACS Alloystructural hardening conductivity Unalloyed copper Cu 320 100Composition as per standards tensile strength 350 Copper and cadmiumCuCd1 480 86 1% cadmium Copper with silver CuAg 320 100 0.08% silverCopper with tin CuSn0.12 Tensile strength 400 88 0.12% tin Copper withchromium 450 80 0.5 to 1% chromium Quenched, annealed, work hard- enedCopper iron 2.3 phosphorus 0.03 415 65 2.3% iron, 0.03% phosphorusQuenched, annealed, work hard- ened Copper nickel silicon CuNi2Si 680 351.6 to 2.5% nickel, 0.5 to 0.8% silicon Quenched, annealed, workhardened Copper with beryllium CuBe2 1060 22 2% berrylium Quenched,annealed Copper cobalt beryllium CuCo2Be 440 60 2.5% cobalt, 0.5%beryllium Factory hardened, high conduc- tivity Brass CuZn20 Approx. 40032 20% zinc Brass CuZn35 Approx. 400 28 35% zinc

Note that adding any metal other than silver, for example cadmium ortin, significantly improves the mechanical properties of the copper, butto the detriment of the conductivity.

Thus the document EP 0 526 361 A teaches the provision of a sparkerosion electrode comprising an external metal layer containing zincaround a metal core comprising copper or a copper alloy. One of therequired objects is to obtain a high mechanical strength of the wire. Itis obvious to the person skilled in the art that the copper used in thiscase is a copper microalloy. The above document further recommendsdoping the copper with one or more elements such as iron, cobalt,titanium, phosphorus, manganese, chromium, zirconium, aluminum, tin,nickel. The document also recommends using alloys, and the only exampleprovided in the document is a wire whose core is of CuZn20 brass.

The document U.S. Pat. No. 4,977,303 A teaches the production of a wirewith a copper core coated with zinc and then subjected to heat treatmentto cause the zinc to diffuse into the copper. In this document, theperson skilled in the art will realize that the copper used is notunalloyed pure copper, since FIG. 4 represents a concentration ofcopper, from a depth of 11 microns, in the core beyond the diffusionlayer, that is clearly less than 100%, while the concentration of zincis zero.

The document US 2001/0050269 A deprecates the use of copper alone in thecore, because of its insufficient mechanical strength at hightemperatures.

None of the above documents describes or suggests using unalloyedcopper, i.e. copper of very high purity.

The present invention is the result of research seeking to optimize thestructure of a spark erosion wire, in order to obtain a high rate oferosion.

With this in view, a first observation drawn from the document EP 0 185492 A is that increasing the thickness of a zinc alloy coating on acopper-plated steel core is beneficial to the rate of spark erosion, butnot beyond a thickness of 15 μm for a total diameter of 200 μm.

The document EP 0 526 361 A previously cited seeks a long electrode lifecombined with a good surface quality of the machined part. The documentteaches increasing the thickness of the surface metal layer with thediameter of the wire. For a 1 mm diameter wire, the thickness of thesurface layer is preferably from 10 to 100 microns. This corresponds toa relative thickness of the surface layer from 1 to 10%. The onlyexample given in the document is a wire whose total diameter is 0.25 mmand comprises a metal surface layer 20 microns thick, which is arelative thickness of 8%. There is no teaching, in the above document,of providing a relative thickness of the surface layer greater than 10%of the diameter of the electrode wire.

A second observation is that, on some spark erosion machines, themachining rate may sometimes be further increased if the metal of thesurface layer is brass obtained by thermal diffusion of zinc on theoutside into an underlying layer containing copper.

This observation stems from the document U.S. Pat. No. 4,977,303 A,which proposes a spark erosion wire in which a copper alloy ormicroalloy core (see FIG. 4) is coated with a thick layer of a copperand zinc alloy obtained by thermal diffusion followed by wire drawing.The diffused alloy layer of copper and zinc is covered with an oxidelayer approximately one micron thick. The document indicates an absolutethickness of the surface metal layer, equal to 22 microns, but gives noindication as to the relative thickness of the surface layer compared tothe diameter of the wire.

In wires with a surface layer of α and β phase diffused zinc and copperalloy, it is however found that increasing the thickness of the surfacelayer on a brass core containing 37% zinc tends to reduce the machiningrate, which is the opposite of what is required. Thus cutting tests havebeen carried out on a 50 mm steel part, firstly with a wire having ahomogeneous brass structure containing 37% zinc, and secondly with awire having a brass core containing 37% zinc covered with a surfacelayer of zinc and copper alloy produced by diffusion heat treatment. Thediameters of the wires and machining conditions being identical, therelative machining rates (in mm²/min) were respectively in theproportions of 98 for the homogeneous wire and 67 for the wire with asurface layer, demonstrating the negative effect of the surface layer.

It was also found that increasing the zinc content of the surface layerimproved spark erosion efficiency. The surface layer-then comprises a βphase, or even a γ phase, which is harder and more rigid. However, it isnot possible to increase the thickness of the surface layer, as the wirebecomes brittle and difficult to draw, especially if the core is ofunalloyed copper.

What is more, until now there has been no benefit in increasing thethickness of the surface metal alloy layer beyond the size of thecraters that spark erosion machines produce in the surface layer of thespark erosion wire during machining. The size of these craters isapproximately 5 microns, as indicated in the document U.S. Pat. No.4,977,303. It has therefore been impossible before now to realize thatit may be beneficial to increase the thickness of the surface layerbeyond a relative thickness of 10% of the diameter of the usual wires,and even less so to realize that it may be beneficial to combine a fixedsurface layer with an unalloyed copper core.

SUMMARY OF THE INVENTION

The problem addressed by the present invention is that of designing anew spark erosion electrode wire structure that significantly increasesthe spark erosion machining rate, for a given diameter, and under givenmachining conditions.

An object of the invention is to propose a method of fabricating thiskind of electrode wire, and a machining method that increases themachining rate.

To achieve the above and other objects, the invention starts from thesurprising observation that, if the core is of unalloyed copper, anincrease in the relative thickness of the diffused brass surface layerproduces a significant increase in the machining rate. The inventiontherefore provides a spark erosion machining electrode wire, comprisinga metal core coated with a coating layer of diffused zinc alloy, inwhich:

-   -   the core is of unalloyed copper,    -   the coating layer is of diffused copper and zinc alloy,    -   the relative thickness of the coating layer of copper and zinc        alloy is greater than 10% of the diameter of the electrode wire.

This kind of spark erosion electrode structure is particularly welladapted to use with spark erosion machines whose electrical generatorsdeliver a higher electrical power, enabling the benefit of the presenceof a thicker surface layer to be obtained.

For example, good results may be obtained for an electrode wire diameterD of 0.20 mm, with a coating layer thickness E greater than or equal to20 microns; for an electrode wire of diameter D equal to 0.25 mm, thethickness E of the coating layer may advantageously be greater than orequal to 25 microns; for an electrode wire diameter D of 0.30 mm, thethickness E of the coating layer may advantageously be greater than orequal to 30 microns; for an electrode wire of diameter D equal to 0.33mm, the thickness E of the coating layer may advantageously be greaterthan or equal to 33 microns; and for an electrode wire of diameter Dequal to 0.35 mm, the thickness E of the coating layer mayadvantageously be greater than or equal to 35 microns. In all cases, anincrease in the spark erosion rate of approximately 30% is observed,compared to a brass or zinc-plated brass wire of the same diameter.

The copper constituting the core is unalloyed copper, the purity ofwhich is defined in French standard NF A 51 050. According to theinvention, the copper is preferably selected from the following familyof recommended coppers, designated by the references used in Frenchstandard NF A 51050, with the corresponding ISO references inparentheses: Cu-a1 (Cu-ETP); Cu-a2 (Cu-FRHC); Cu-C1 (Cu-OF); Cu-c2(Cu-OFE).

In practice, the unalloyed copper may be selected as a function of itselectrical conductivity. The recommended unalloyed copper has anelectrical conductivity of approximately 100% IACS, i.e. 58MegaSiemens/meter at 20° C. At 20° C., the electrical conductivity ofthe unalloyed copper core, work hardened as a result of wire drawing, isof the order of 99% IACS.

The high electrical conductivity of the unalloyed copper corework-hardened as a result of wire drawing prevents excessive heating ofthe electrode wire during spark erosion, and thus protects it frombreaking, unlike copper microalloys.

A second aspect of the invention highlights the influence of the overallconductivity of the electrode wire on spark erosion performance, andexploits this influence to increase the machining rate on the assumptionthat electrical energy will be supplied by more and more powerfulgenerators.

The overall electrical conductivity of the electrode wire is the sum ofthe conductivities of the core and the coating layer, multiplied bytheir respective areas in the section of the wire. The electrode wireaccording to the invention has an electrical conductivity of at least60% IACS (60% of the normalized conductivity of annealed pure copper).Failing this, a progressive reduction of the spark erosion rate isobserved.

To be more precise, it has been observed that the overall electricalconductivity of the electrode wire may advantageously be from 65% IACSto 75% IACS.

Below 65% IACS, optimum spark erosion cutting performance is notachieved because of the insufficient conductivity of the electrode wire.The wire breaks more easily as a result of heating in the sparking area.This is caused by the more intense Joule effect and by reduced coolingassociated with the lower thermal conductivity.

The required type of electrode wire cannot be obtained above 75% IACS,because it is then obligatory to reduce the thickness of the diffusedlayer below 10% of the diameter of the electrode wire. Failing this, thewire is too rigid and brittle, and must not be drawn during itsfabrication.

The recommended overall electrical conductivity of the electrode wire isof the order of 69% IACS, and corresponds to a diffused layerapproximately 35 μm thick for a 0.33 mm electrode wire, i.e. a relativethickness of approximately 11%. In this case, the coefficient β ofvariation of the overall resistivity of the electrode wire relative totemperature is 0.0034° K⁻¹. It must be remembered that the resistivityR(T) of a wire is affected by temperature in accordance with the lawR(T)/R₀=1+β(T−T₀), where R(T) is the resistivity of the wire at thetemperature T concerned, and R₀ is its resistivity at the referencetemperature T₀.

The relative thickness values of 11% and overall electrical conductivityvalues of 69% IACS give good results in the range of wire diameters fromapproximately 0.20 mm to approximately 0.35 mm.

Two parameters are available to the operator to obtain the aboveconductivity values during fabrication of the electrical wire: thethickness of the layer of zinc initially deposited, and the extent ofthe heat treatment producing diffusion of the zinc and the copper. Theoperator will have no problem in making an appropriate choice of thesetwo parameters.

The above considerations and overall electrical conductivity values havebeen applied successfully to the production of electrode wires with asurface layer of zinc and copper alloy whose thickness is greater thanor equal to 10% of the diameter, on an unalloyed copper core.

They may also be applied with advantage to the production of electrodewires of different structure, for example with a thinner surface layer,a surface layer of other metals or alloys, multiple surface layers, onan unalloyed copper core or a core of another metal or alloy.

The advantageous and unexpected properties of wires according to theinvention have been verified by experiment. A Charmilles Robofil 2020machine was used for comparative machining of 50 mm high parts made fromZ 160 CDV12 steel using the following wires, all of which had the samediameter (0.25 mm):

Number Core Layer, thickness Conductivity (% IACS) 1 Unalloyed copperDiffused, 5% 82 2 Unalloyed copper Diffused, 11% 67 3 Unalloyed copperDiffused, 16% 63 4 Copper with mag- Diffused, 11% 61 nesium 5 Copperwith iron, Diffused, 11% 45 phosphorusThe spark erosion rate and mechanical tensile strength of each wire weretested simultaneously by carrying out machining under conditions thatwere made increasingly more difficult by progressively reducing thepressure of injection of water into the machining area.

Wire 1, with a thinner unalloyed copper surface layer, provided amachining rate of 145 mm²/min at the maximum water injection pressure,and broke when the water injection pressure was below approximately 3.2bar.

Wire 2 in accordance with the invention, with an 11% thick unalloyedcopper surface layer, produced a machining rate greater than 168mm²/min, and broke when the water injection pressure was lower thanapproximately 4 bar.

Wire 3, with a 16% thick unalloyed copper surface layer, produced ahigher machining rate of 171 mm²/min, but broke as soon as the waterinjection pressure was below approximately 8 bar. A 16% surface layer ofthis kind may be deemed to constitute an upper limit that it is betternot to exceed.

Wires 4 and 5, with an alloyed copper core, produced machining rates of165 mm²/min and 161 mm²/min, respectively, but broke as soon as thewater injection pressure was lower than approximately 5 bar.

The above tests demonstrate the advantageous and unexpected propertiesof wires according to the invention: in a way that was not obvious, thewires with alloyed copper cores, normally better at high temperature,were weaker than wires with an unalloyed copper core when machiningunder unfavorable cooling conditions.

The fabrication of an electrode wire as defined hereinabove may comprisethe following steps:

a. providing an unalloyed copper core wire of diameter greater than thediameter of the wire to be produced,

b. covering the core wire with pure zinc to an appropriate thickness,

c. subjecting the coated core wire to diffusion heat treatment to form acoating layer,

d. drawing the electrode wire to the final diameter, the coating layerthen having a thickness greater than 10% of the final diameter of theelectrode wire.

During step b, the zinc is preferably deposited on the copper core wireelectrolytically.

After the diffusion step (c) or after the drawing step (d), theelectrode wire may further be covered with a thin contact surface layer,for example of zinc, copper, nickel, silver or gold. This may beachieved by electrolytic deposition in particular.

In accordance with the invention, an electrode wire as defined above mayadvantageously be used for spark erosion machining a part. In this case,in a machine employing an electrical generator to produce the sparkingelectrical energy, the generator is set to produce the maximum sparkingenergy compatible with the machining capacity of the electrode wirewithout breaking, thereby increasing the machining rate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willemerge from the following description of particular embodiments of theinvention, given with reference to the appended drawings, in which:

FIG. 1 is a diagrammatic front view of a spark erosion machine of thetype using a wire

FIG. 2 is a plan view showing the process of spark erosion in the FIG. 1machine;

FIG. 3 is a plan view of the machined part from FIGS. 1 and 2;

FIG. 4 is a diagrammatic perspective view to an enlarged scale of oneembodiment of an electrode wire of the present invention; and

FIG. 5 is a diagrammatic view in cross section of a preferred embodimentof an electrode wire of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Consider firstly FIGS. 1 to 3, which depict spark erosion machiningusing an electrode wire. The spark erosion machine shown in FIG. 1essentially comprises a machining enclosure 1 containing a dielectricsuch as water, means such as pulleys 2 and 3 and wire guides 20 and 30for holding an electrode wire 4 and tensioning it in a sparking area 5inside the enclosure 1, a work support 6, and means 7 for moving thework support 6 relative to the electrode wire 4 in the sparking area 5.The part 8 to be machined, held by the work support 6, is placed in thesparking area 5. The wire guides 20, 30 are on either side of the part 8to be machined, and guide the electrode wire 4 accurately. To this endthey are positioned close to the part 8 to be machined, and theirdiameter is only slightly greater than that of the electrode wire 4, forexample a diameter of 254 μm for an electrode wire 4 of 250 μm diameter.The electrode wire 4 is moved longitudinally in the sparking area 5 andfacing the part 8 to be machined as indicated by the arrow 9. Anelectrical general 10, electrically connected, on the one hand, to theelectrode wire 4 by a line 18 and to a contact 18 a that touches theelectrode wire 4 when it enters the dielectric in the enclosure 1between the pulley 2 and the wire guide 20, and, on the other hand,connected to the part 8 to be machined by a line 19, generates in thesparking area 5 electrical energy appropriate to cause electrical arcsto be struck between the part 8 to be machined and the electrode wire 4.

The machine comprises control means for adapting the electrical energy,the speed at which the electrode wire 4 moves, and the displacement ofthe part 8 to be machined as a function of the machining steps.

As can be seen in FIG. 2, by moving the part to be machined in atransverse direction shown by the arrow 11, the spark erosion processcauses the electrode wire 4 to penetrate progressively into the mass ofthe part 8 to be machined which is electrically conductive, and producesa slot 12. Then, by moving the part 8 to be machined in the direction ofthe arrow 13, a perpendicular cut is produced, finally yielding a partas shown in FIG. 3, with a first machined facet 14 and a second machinedfacet 15.

Obviously generating high electrical energy by means of the electricalgenerator 10 enables fast sparking and therefore faster movement of thepart to be machined relative to the electrode wire 4, for fastmachining. In fact, the movement of the part must track the erosionproduced by the sparks, without excess. An excessively low speed reducesthe machining rate. An excessively high speed causes contact of the wireand the part, and the resulting short circuit stops the machine.

However, the electrical energy heats the wire in the machining area, andincreasing this energy simultaneously increases the risks of the wirebreaking. Accordingly, for a given structure of the electrode wire, themaximum machining rate is obtained for an electrical energy just belowthe energy that would cause the electrode wire to break.

Consider now again the tests that led to the idea of the presentinvention.

Spark erosion machining tests were carried out on a steel part 50 mmhigh in a Charmilles Robofil 2020 machine using cutting setting E3.

A first comparative test was carried out, on the one hand, with a brasselectrode wire containing 37% zinc, and, on the other hand, with anelectrode wire having a brass core containing 37% zinc covered with an 8micron layer of an α and βphase alloy of copper and zinc obtained bydiffusion heat treatment. The two electrode wires had the same finaldiameter of 0.25 mm. The brass electrode wire achieved a relativemachining rate of 98, whereas the electrode wire with a brass corecovered with diffused zinc and copper alloy achieves a relativemachining rate of only 67.

A second comparative test was carried out using, on the one hand, anelectrode wire whose core was of copper and zinc alloy containing 80%copper, with a 20 micron coating layer of an α and β phase diffused zincand copper alloy, and, on the other hand, with an electrode wire with anunalloyed copper core coated with a 14 micron layer of diffused zinc andcopper alloy. The two electrode wires achieved relative machining ratesof 109 and 125, respectively. This demonstrates the advantage of anunalloyed copper core, which machines faster than the brass core even ifthe coating layer is thinner.

A third test employed in succession three 0.25 mm diameter electrodewires having an unalloyed copper core, with coating layers of diffusedzinc and copper alloy 11 microns, 14 microns, and 28 microns thick,respectively. The relative machining rates obtained were 115, 125 and133, respectively. It is seen that, for the same sparking power, athicker diffused layer accelerates cutting, in the case of electrodewires with unalloyed copper cores.

Exploiting these results, the invention achieves a higher machining rateby using a particular electrode wire, shown to a larger scale in FIG. 4.The electrode wire according to the invention comprises an unalloyedcopper core 16, coated with a layer 17 of diffused zinc and copper alloywhose thickness E is greater than 10% of the diameter D of the electrodewire.

It may be beneficial to increase significantly the thickness E of thecoating layer. However, a limit is encountered in the relativedeformation capacity of the metals during drawing to obtain the requiredfinal dimension of the electrode wire: too great a thickness of thecoating layer leads to the risk of the wire breaking during drawing,which affects the production and use properties of the electrode wire.It remains easy to carry out drawing if the relative thickness of thecoating layer is less than approximately 16% of the final diameter D. Atthe same time, too great a thickness E of the coating layer makes thewire brittle because of insufficient electrical conductivity.

The interface between the core and the coating layer is generallydeformed by the wire drawing operation, which naturally eliminates itssmooth nature and makes it slightly irregular. This irregularity is nota problem for the spark erosion process.

A contact surface layer 21 may advantageously be added to the electrodewire, for example of zinc, copper, nickel, silver or gold, to improveelectrical conduction between the electrode wire 4 and the contact 18 a,and make sparking more stable.

A thick layer of copper significantly reduces the spark erosion rate. Toprevent this drawback, the copper layer must be extremely thin, forexample less than 0.5 micron thick.

A layer of nickel appears to be too fragile to be continuous atthicknesses of the order of 1 micron.

A zinc layer of approximately 1 micron is beneficial. Even ifdiscontinuous, this layer unexpectedly improves the electrical contactand the stability of sparking.

The surface of the electrode wire may be covered with a thin layer ofoxide, resulting from fabrication process steps. It is not essential toeliminate this layer, although it is possible. This layer may be uniformor non-uniform.

The surface of the electrode wire may be cracked, without this reducingthe machining rate.

The electrode wire obtained in accordance with the invention isgenerally yellow-brown in color.

The surface of the electrode wire must be relatively clean, with fewtraces of wire drawing lubricants or other soiling.

For an electrode wire as defined hereinabove, improved spark erosionproperties are still obtained if the coating layer is an alloy of copperand zinc with a heterogeneous mixture of α and β and/or β′ phases. Thezinc content by weight is then from 35% to 57%, preferably from 35% to50%. Tested electrode wires, that proved satisfactory, had measured zinccontents of 45.7%, 41.5%, and 35.4%, and an oxygen content of 0.5% inthe form of zinc or copper oxides. The phases present in the surfacelayer were the α and β and β′ phases of the copper-zinc diagram.

FIG. 5 shows diagrammatically in cross section the structure of thesurface layer of a preferred embodiment of a wire of the invention. Thestructure is heterogeneous, in the sense that some portions of thesurface layer are crystallized as phase β or β′, from the core as far asthe external surface, while other areas consist of a mixture of onephase in a matrix of another phase.

There can therefore be made out, in the figure, the unalloyed coppercore 16, and the coating 17 of copper and zinc alloy whose thickness isgreater than 10% of the diameter. The area 17 a is of big β phasecrystal, which may have a size T from a few microns to more than 10microns. The area 17 b is an area of mixed phases α and β, for example,as shown to a larger scale in the box in the top right-hand corner ofthe figure, microzones of phase β, from 1 micron to a few microns forexample, distributed in a matrix of the α phase. Conversely, in the area17 c, microzones of the a phase are found distributed in a matrix of theβ phase. The area 17 d is a combination of a surface layer of the βphase and a lower layer of mixed α and β phases.

This kind of heterogeneous structure is obtained by an appropriatechoice of heat diffusion conditions during the production of the coatinglayer: fast heating, appropriate diffusion time.

Heating is continued just long enough to obtain the required mixture ofphases.

The benefits of this structure especially include facilitating wiredrawing, despite the a priori unfavorable presence of the β phase, withthe result that it is then possible to increase the zinc content andconsequently to increase the spark erosion rate.

An electrode wire according to the invention may be produced by a methodcomprising the following steps:

a. providing an unalloyed copper core wire of diameter D1 greater thanthe diameter D of the wire to be produced,

b. covering the core wire with pure zinc to an appropriate thickness toproduce afterwards the final thickness;

c. subjecting the coated core wire to diffusion treatment, to form acoating layer 17;

d. drawing the electrode wire to the final diameter D, the coating layer17 then having a thickness E greater than 10% of the final diameter D ofthe electrode wire.

The present invention is not limited to the embodiments explicitlydescribed and encompasses variants and generalizations thereof containedwithin the scope of the following claims.

1. A spark erosion machining electrode wire comprising a metal corecoated with a coating layer of diffused zinc alloy, wherein: the core isof unalloyed copper, the coating layer is of diffused copper and zincalloy, the thickness of the coating layer of copper and zinc alloy isgreater than about 10% of the diameter of the electrode wire, and theoverall electrical conductivity of the electrode wire is from about 65%IACS to about 75% IACS.
 2. Electrode wire according to claim 1, whereinthe overall electrical conductivity of the electrode wire is of theorder of 69% IACS.
 3. Electrode wire according to claim 1, wherein thecoating layer is of copper and zinc alloy with a mixture of α and βphases, α and β′ phases, or α, β, and β′ phases, and the zinc content byweight of the coating layer is from about 35% to about 50%.
 4. A methodof producing a spark erosion electrode wire, said wire comprising ametal core coated with a coating layer of diffused zinc alloy, wherein:the core is of unalloyed copper, the coating layer is of diffused cooperand zinc alloy, and the thickness of the coating layer of copper andzinc alloy is greater than about 10% of the diameter of the electrodewire, said method comprising the following steps: a. providing anunalloyed copper core wire of diameter greater than the diameter of thespark erosion electrode wire to be produced, b. coating the core wirewith pure zinc to form a coated core wire, c. subjecting the coated corewire to diffusion heat treatment to form a coating layer, d. drawing thecoated core wire to the final diameter, the coating layer then having athickness greater than 10% of the final diameter of the spark erosionelectrode wire, wherein the overall electrical conductivity of the sparkerosion electrode wire is from about 65% IACS to about 75% IACS. 5.Method according to claim 4, wherein: for an electrode wire diameter of0.20 mm, the thickness of the coating layer is greater than or equal to20 microns, for an electrode wire diameter of 0.25 mm, the thickness ofthe coating layer is greater than or equal to 25 microns, for anelectrode wire diameter of 0.30 mm, the thickness of the coating layeris greater than or equal to 30 microns, for an electrode wire diameterof 0.33 mm, the thickness of the coating layer is greater than or equalto 33 microns, and for an electrode wire diameter of 0.35 mm, thethickness of the coating layer is greater than or equal to 35 microns.6. A method for machining a part by spark erosion in a machine employingan electrical generator to produce the sparking electrical enemy, themethod comprising generating sparks between an electrode wire and thepart, producing relative movement of the electrode wire and the parttransversely to the longitudinal direction of the electrode wire, anderoding the part, wherein: the generator is set to produce the maximumsparking energy compatible with the machining capacity of the electrodewire without breaking the electrode wire, thereby increasing themachining rate, and the electrode wire is a spark erosion machiningelectrode wire comprising a metal core coated with a coating layer ofdiffused zinc alloy, wherein: the core is of unalloyed cooper, thecoating layer is of diffused copper and zinc alloy, and the thickness ofthe coating layer of copper and zinc alloy is greater than about 10% ofthe diameter of the electrode wire, wherein the overall electricalconductivity of the electrode wire is from about 65% IACS to about 75%IACS.